As a method for obtaining a large screen image, it is well known conventionally that an optical image is formed on a reflection type light valve in accordance with a video signal, and the optical image is irradiated with light to be projected on a screen in a magnified state by a projection lens system. If a reflection type light distribution correction element for forming an optical image by controlling the traveling direction of light in accordance with a video signal is used as the reflection type light valve, a projected image with a more efficient light use and a higher brightness can be displayed.
As the reflection type light valve, a DMD (Digital Micro Mirror Device) is being given attention. The DMD has a configuration in which a plurality of minute reflection mirrors (hereinafter, referred to as “micro mirrors”) are disposed two-dimensionally on a silicon substrate, and each micro mirror constitutes a pixel. Each micro mirror is configured so as to move like a seesaw in a range of ±10° by two rotation spindles provided in a diagonal direction at a diagonal position of a pixel. For example, it is assumed that the state where a micro mirror is tilted at +10° is ON, and the state where a micro mirror is tilted at −10° is OFF. The DMD tilts each micro mirror at +10° or −10° in accordance with a video signal, thereby controlling the output direction of a light beam to form an optical image.
FIG. 17 shows an operation state of micro mirrors constituting the respective pixels of a conventional DMD. FIG. 17 shows a cross-section taken along a surface perpendicular to a rotation spindle of each micro mirror of the DMD. The counterclockwise direction corresponds to a rotation positive direction of a micro mirror. In FIG. 17, reference numerals 191 to 196 denote micro mirrors that constitute the respective pixels. Reference numeral 197 denotes a part of a projection lens system.
In the example shown in FIG. 17, the micro mirrors 191, 193, and 196 are tilted at +10° (in a counterclockwise direction) with respect to a reference surface 190 of the reflection type light valve (DMD), whereby they are in an ON state. Therefore, incident light 198 reflected from the micro mirrors 191, 193, and 196 is incident upon the projection lens system 197.
On the other hand, the micro mirrors 192, 194, and 195 are tilted at −10° (in a clockwise direction) with respect to the reference surface 190 of the reflection type light valve, whereby they are in an OFF state. Therefore, the incident light 198 reflected from the micro mirrors 192, 194, and 195 is not incident upon the projection lens system 197. Such a DMD has useful characteristics, i.e., it can use natural light, and has a high light use efficiency and a high response speed, compared with a liquid crystal panel using polarized light.
WO 98-29773 shows a configuration example of an optical system of a projection type display apparatus using a DMD as a reflection type light valve. FIG. 18 shows a schematic configuration of a projection type display apparatus using a conventional DMD. FIG. 19 shows a portion in the vicinity of the DMD shown in FIG. 18 in a magnified state. FIGS. 18 and 19 show cross-sections taken along a surface perpendicular to a rotation spindle of each micro mirror of the DMD.
First, a description will be made with reference to FIG. 18. A light source 201 is composed of a concave mirror 201b and a lamp 201a. The concave mirror 201b is an elliptical mirror, which is formed by vapor-depositing an optical multilayer film that transmits infrared light and reflects visible light on an inner surface of a glass substrate. The lamp 201a is disposed so that the center of its illuminator is positioned at a first focal point (not shown) of the concave mirror 201b. 
Light radiated from the lamp 201a is reflected from the concave mirror 201b, and travels to a second focal point (not shown) of the concave mirror 201b to form an illuminator image at the second focal point. Furthermore, the light passing through the second focal point passes through lens arrays 202a and 202b successively to be split into a plurality of luminous fluxes. Thereafter, the luminous fluxes are incident upon a relay lens 203 to be overlapped with each other. The lens arrays 202a and 202b are composed of a plurality of lens elements with a positive power.
The light output from the relay lens 203 is reflected from a total reflection mirror 204 to be incident upon a total reflection prism 208 via a field lens 205. The total reflection prism 208 is composed of two single prisms 208a and 208b spaced by an air layer 209. Reference numeral 207 denotes a projection lens system.
Next, a description will be made with reference to FIG. 19. Incident light 209a to 209c that is incident upon the total reflection prism 208 is totally reflected from an interface between the single prism 208b and the air layer 209 to travel to the reflection type light valve 206. The reflection type light valve 206 controls the traveling direction of light in accordance with a video signal to form an optical image.
Reflected light 210a to 210c from the reflection type light valve 206 is output as luminous fluxes having a principal ray perpendicular to a display region of the reflection type light valve 206, passes through the total reflection prism 208 without being reflected from the interface between the single prism 208b or 208a and the air layer 209, and is incident upon the projection lens system 207 (see FIG. 18). As a result, the optical image on the reflection type light valve 206 is projected on a screen in a magnified state by the projection lens system 207.
Thus, when the projection type display apparatus shown in FIGS. 18 and 19 is used, the optical path of illumination light can be prevented from being overlapped with that of projected light, and the quality of a projected image can be enhanced. Furthermore, the size of the projection lens system can be kept from being enlarged.
However, in the projection type display apparatus shown in FIGS. 18 and 19, the total reflection prism 208 is required for separating light into illumination light and projected light. This leads to an increase in cost. Furthermore, the total reflection prism 208 includes a minute air layer, so that the resolution characteristics of the projection lens system 207 are influenced largely by the tolerance of the air layer.
In order to solve the above-mentioned problem, JP 2000-98272 A discloses a configuration in which a projection lens system is designed as a non-telecentric type, and illumination is generated in accordance therewith.
FIG. 20 shows a schematic configuration of a conventional projection type display apparatus in which a projection lens system is designed as a non-telecentric type. FIG. 21 shows a portion in the vicinity of a reflection type light valve shown in FIG. 20 in a magnified state. In FIGS. 20 and 21, a DMD is used as the reflection type light valve. FIGS. 20 and 21 show a cross-section taken along a surface perpendicular to a rotation spindle of each micro mirror of the DMD.
As shown in FIG. 20, a light source 211 is composed of a lamp 211a and a concave mirror 211b in the same way as the light source shown in FIG. 18. The lamp 211a and the concave mirror 211b are the same as those shown in FIG. 18. The lamp 211a also is disposed so that the center of its illuminator is positioned at a first focal point f1 of the concave mirror 211b. In the same way as the example shown in FIG. 18, light radiated from the lamp 211a is reflected from the concave mirror 211b to form an illuminator image at a second focal point f2. The light passing through the second focal point f2 is incident upon a rod lens 212 to be made uniform. The illumination light that has been made uniform by the rod lens 212 passes through a relay lens 213.
As shown in FIG. 21, the illumination light passing through the relay lens 213 passes through an output pupil 217 of an illumination optical system to be incident upon a reflection type light valve 214 at a predetermined incident angle. The reflection type light valve 214 controls the traveling direction of light in accordance with a video signal to form an optical image. The incident light 215a to 215c to the reflection type light valve 214 is reflected respectively at predetermined angles, and reflected light 216a to 216c is incident upon an entrance pupil 218 of a projection lens system 219.
Furthermore, in the projection type display apparatus shown in FIGS. 20 and 21, a projection lens system of a non-telecentric type is used as the projection lens system 219. Therefore, an optical image formed on the screen by the reflection type light valve 214 can be projected in a magnified state without using a total reflection prism. Thus, it is considered that the cost of the projection type display apparatus shown in FIGS. 20 and 21 can be decreased more than that of the projection type display apparatus shown in FIG. 18.
The reflection type light valve 214 is configured so that the normal directions of the reflection surfaces of micro mirrors become constant over the display region. Therefore, in the configuration of the projection type display apparatus shown in FIGS. 20 and 21, when the optical axis of the reflection type light valve 214 is substantially matched with that of the projection lens system, the optical paths of the incident light 215a to 215c are overlapped with those of the reflection light 216a to 216c. Because of this, as shown in FIGS. 20 and 21, the optical axis of the projection lens system 219 is offset from that of the reflection type light valve 214, whereby the incident light 215a to 215c is separated from the reflected light 216a to 216c. 
However, in the projection type display apparatus shown in FIGS. 20 and 21, the projection lens system 219 projects an image with its optical axis shifted from the optical axis of the reflection type light valve 214. Therefore, in order to obtain a satisfactory image with uniform illumination, it is required to enlarge an effective display region. Consequently, the projection type display apparatus shown in FIGS. 20 and 21 has a problem in that an optical system is enlarged, resulting in an increase in cost. There also is a problem in that front projection cannot be performed.
Furthermore, JP 11(1999)-249069 A discloses a projection type display apparatus in which a condenser lens constituting a part of a projection lens system is disposed immediately before a display region of a reflection type light valve. In this projection type display apparatus, illumination light is refracted by the condenser lens to be incident upon the reflection type light valve, and output light from the reflection type light valve also is refracted by the condenser lens to be incident upon the projection lens system. Furthermore, the lens is disposed with its optical axis deflected from that of the projection lens system.
Therefore, the incident angle of incident light to the reflection type light valve and the output angle of output light from the reflection type light valve are changed in accordance with the position of the display region of the reflection type light valve, and the changes in the incident angle and the output angle become non-symmetrical with respect to the optical axis of the reflection type light valve or that of the projection lens system.
Thus, even in the projection type display apparatus described in JP 11(1999)-249069 A, the overlap of the optical path of the incident light to the reflection type light valve and the optical path of the output light from the reflection type light valve can be reduced. Furthermore, since it is not required to use a prism, an apparatus can be miniaturized.
However, in the projection type display apparatus described in JP 11(1999)-249069 A, the condenser lens is disposed immediately before the display region of the reflection type light valve is deflected, and the condenser lens constitutes a part of the projection lens system. Therefore, it is considered to be difficult to obtain an image in which an aberration balance becomes symmetrical with respect to the optical axis. Furthermore, when it is attempted to correct the aberration balance, it is necessary to increase the number of projection lenses, which makes the projection lens system complicated.
Furthermore, in the projection type display apparatus described in JP 11(1999)-249069 A, in order to obtain a satisfactory resolution, the reflection type light valve is tilted at 2° to 8° with respect to the optical axis of the projection lens system. However, according to the “Shineproof Theorem”, it is considered that a projected image of the reflection type light valve also is tilted with respect to the optical axis of the projection lens system. Therefore, in the case where the display region of the reflection type light valve is in a rectangular shape, the projected image on a surface perpendicular to the optical axis has a trapezoidal shape; accordingly, it may be difficult to obtain a satisfactory display image. The Shineproof Theorem refers to a theorem: when an object is tilted with respect to an optical axis, an image is tilted in a reverse direction, and these tilt angles can define each other.
Furthermore, JP 2000-39585 A also discloses a projection type display apparatus having a configuration in which a positive lens is disposed immediately before the display region of the reflection type light valve, in the same way as in JP 11(1999)-249069 A. Even in the projection type display apparatus, illumination light from an illumination optical system passes through a positive lens, so that it illuminates the reflection type light valve after being refracted. Furthermore, output light from the light valve is incident upon a projection lens system after being refracted by a positive lens.
In the projection type display apparatus described in JP 2000-39585 A, a partial region of the effective region of a positive lens is used for allowing illumination light to pass therethrough, and the remaining region is used for allowing reflected light from the reflection type light valve to pass therethrough. Therefore, the optical axis of the positive lens is largely shifted from that of the main group of the projection lens system.
Therefore, even in the projection type display apparatus described in JP 2000-39585 A, the overlap of the optical path of the incident light to the reflection type light valve and that of output light from the reflection type light valve can be reduced, whereby these optical paths can be separated from each other. Furthermore, since it is not required to use a prism, an apparatus can be miniaturized.
However, even in the projection type display apparatus disclosed in JP 2000-39585 A, the reflection type light valve is disposed so that its optical axis forms an angle of 5° to 15° with respect to the optical axis of the main group of the projection lens system. Thus, the optical axis of the projected image and that of the light valve are not parallel to the optical axis of the projection lens system.
Therefore, even in the projection type display apparatus disclosed in JP 2000-39585 A, in the same way as the projection type display apparatus disclosed by JP 11(1999)-249069 A, a projected image is tilted to have a trapezoidal shape in accordance with the “Shineproof Theorem”, and it may be difficult to obtain a satisfactory image.
Furthermore, the positive lens disposed immediately before the display region of the reflection type light valve is disposed so that its optical axis forms an angle with respect to the optical axis of the light valve. Furthermore, it is required to dispose a deflection lens in the projection lens system. Therefore, even in the projection type display apparatus disclosed by JP 2000-39585 A, in the same way as the projection type display apparatus disclosed by JP 11(1999)-249069 A, it is considered to be difficult to obtain an image in which an aberration balance becomes symmetrical with respect to the optical axis. Furthermore, when it is attempted to correct the aberration balance, it is required to increase the number of projection lenses, which makes the projection lens system complicated. Furthermore, in this case, when a positive lens is composed of a double-convex lens, the central thickness of the lens is increased. When the positive lens is composed of a meniscus lens, it is difficult to obtain a sufficient power.
In the projection type display apparatus disclosed by JP 2000-39585 A, a part of illumination light incident upon a positive lens is reflected from an interface between the positive lens and the air layer due to the difference in refractive index therebetween. Furthermore, as the positive lens, a double-convex lens or a lens with a convex surface placed on the projection lens system side and a concave surface placed on the reflection type light valve side is used.
Because of this, the reflected light reflected from the interface is reflected in a main group direction of a projection lens system to reach a screen. The reflected light reflected from the interface is stray light generated constantly irrespective of a video signal input to the reflection type light valve. The stray light causes a decrease in contrast in a projected image and generation of a ghost image, which decrease the quality of the projected image remarkably.
In general, a reflection preventing film having practically sufficient performance reflects at least about 0.5% of the incident light, and transmits at most about 99.5%. Therefore, it is considered that if a reflection preventing film at an ordinary level is formed by stacking a TiO2 film, an SiO2 film, and the like on the surface of the positive lens, the reflected light possibly can be decreased. However, there is a limit to the reduction of reflected light only by forming the reflection preventing film composed of such a multilayer film, and hence, there also is a limit to the improvement of the quality of the projected image. Furthermore, it is ideal that a reflection preventing film transmitting 100% incident light may be formed; however, at present, it is practically impossible to form such a reflection preventing film.
The object of the present invention is to provide: a small projection type display apparatus that overcomes the above-mentioned problems and is capable of obtaining a projected image of high quality by suppressing the optical path of incident light to a reflection type light valve from being overlapped with the optical path of output light from the reflection type light valve, and suppressing unnecessary reflected light in a lens interface from being incident upon the projection lens system; and a rear projector and a multi-vision system using the projection type display apparatus.